With the recent demand for a higher speed and broader band of a network, the 40-Gbps line has begun to be introduced also for the wavelength multiplex propagation apparatus. In the wavelength multiplex propagation apparatus, a waveform is deformed through the propagation path having a dispersion value, and therefore, dispersion compensation to shape the waveform is required with a reception unit equipped with a dispersion medium having a characteristic inverse to the propagation path.
FIG. 11 is a diagram illustrating an example of the dispersion characteristic of various fibers. As shown in FIG. 11, the fiber dispersion coefficient varies with the type and the wavelength of each fiber. In order to pass the signal, the residual dispersion (the dispersion value excluding the compensation value of the dispersion compensation from the dispersion value of the propagation line) in the optical/electrical conversion unit is required to be included in a predetermined range (dispersion tolerance). FIG. 12 is a diagram illustrating the relation between the propagation distance and the residual dispersion of various fibers.
In the conventional wavelength multiplex propagation apparatus, the transmission of the 10-Gbps signal is accompanied by the line design to compensate for the dispersion value of the line collectively for the multiplexed signals. FIG. 13 is a diagram (1) illustrating the configuration of the conventional wavelength multiplex propagation apparatus. As illustrated in FIG. 13, the wavelength multiplex propagation apparatus 10 includes a dispersion compensator 11, an amplifier 12, an optical demultiplexer 13, and optical receiving units 14a to 14c. 
In FIG. 13, the wavelength multiplex propagation apparatus 10 is such that the dispersion compensator 11 compensates for the dispersion collectively for the wavelength-multiplexed signals, and the signal with the dispersion compensated is output to the amplifier 12. The wavelength-multiplexed signal, after being amplified by the amplifier 12, is separated into various wavelengths (λ1 to λn) by the optical demultiplexer 13 and output to the optical receiving units (OR units) 14a to 14c. 
In the propagation of the 40-Gbps signal, however, the time per bit is shorter than in the propagation of the 10-Gbps signal, and therefore, the dispersion compensation higher in accuracy is required. In order to ensure this dispersion compensation of higher accuracy, the line design has been employed to include a tunable dispersion compensator for each wavelength. FIG. 14 is a diagram (2) illustrating the configuration of the conventional wavelength multiplex propagation apparatus. In FIG. 14, the wavelength multiplex propagation apparatus 20 includes a dispersion compensator 21, an amplifier 22, an optical demultiplexer 23, tunable dispersion compensators 24a to 24c, and optical receiving units 25a to 25c. The tunable dispersion compensators 24a to 24c and the optical receiving units 25a to 25c are combined as receivers 26a to 26c, respectively.
In FIG. 14, the dispersion compensator 21 of the wavelength multiplex propagation apparatus 20 compensates for the dispersion collectively for the wavelength-multiplexed signal, and outputs the dispersion-compensated signal to the amplifier 22. The wavelength-multiplexed signal, after being amplified by the amplifier 22, is demultiplexed by the optical demultiplexer 23 for each wavelength. The resulting signal of each wavelength is input to the corresponding tunable dispersion compensators 24a to 24c, and after the dispersion is compensated for each wavelength, is output to the corresponding optical receivers (OR) 25a to 25c. 
FIG. 15 is a diagram illustrating the configuration of the receiving unit 26a (the receiving units 26b and 26c are also similar). As illustrated in FIG. 15, the receiving unit 26a includes a tunable dispersion compensator (TDC) 30, an optical/electrical converter (O/E) 31, a frame detection unit 32, an error detection unit 33, and a TDC control unit 34.
In building up a line, the signal (optical signal) dispersion-compensated by the TDC 30 is converted into an electrical signal by the O/E 31 of the receiving unit 26a, and a frame (a frame containing an error code) is detected from the electrical signal by the frame detection unit 32.
Then, the error detection unit 33 detects a frame error based on the error code included in the frame, and outputs the detection result (number of errors detected or number of errors corrected) to the TDC control unit 34. The TDC control unit 34 scans the TDC 30 and sets the optimum dispersion value to minimize the number of errors detected by the error detection unit 33.
FIG. 16 is a diagram illustrating an example of the configuration of the TDC 30. In FIG. 16, a virtually imaged phased array (VIPA) is shown as an example of the TDC 30. As shown in FIG. 16, the TDC 30 includes an optical circulator 40, a collimator lens 41, a line focus lens 42, a VIPA glass plate 43, a focus lens 44, and a mirror 45.
The TDC 30, in accordance with a control instruction from the TDC control unit 34, moves the mirror 45 in parallel to adjust the dispersion value. By adjusting the dispersion value optimally in accordance with the signal, the number of errors detected by the error detection unit 33 can be suppressed. The parallel movement of the mirror 45 toward the minus side reduces the dispersion value, while the parallel movement thereof toward the plus side increases the dispersion value.
When the wavelength multiplex propagation apparatus 20 newly builds a line of a new wavelength, the optimum initial value of the dispersion value (hereinafter referred to as the initial dispersion value) is desirably set in the TDC 30. This is by reason of the fact that if a wrong initial dispersion value is set in the TDC 30, the TDC 30 would be required to continue to be scanned until the number of detected errors is converged into a tolerable range, thereby taking a long time before building the line.
In view of this, various techniques for calculating the initial dispersion value of a new wavelength have been proposed. Japanese Patent Application Laid-Open No. 2008-72555, for example, discloses the method in which the dispersion value set in the existing wavelength (the wavelength of the line already built) closest to the new wavelength is used directly as the initial dispersion value of the new wavelength.